Traditional incandescent light bulbs generate a wide range of uniform light from a filament by supplying electric power from an external power source to the filament. However, the incandescent light bulbs have disadvantages such that power consumption is high, temperature is high, and lifespan is short.
In contrast, since light-emitting diode (LED) bulbs have advantages such that lifespan is long, power can be saved, environmental pollution caused by waste does not occur, the LED bulbs are becoming lighting fixtures of a new age instead of the incandescent light bulbs. However, since the LEDs emit light only in a forward direction, when an LED bulb of the related art is attached to a ceiling, the light that is radiated to the ceiling and a wall surface decreases. Therefore, as compared to the incandescent light bulbs in which equivalent illuminance is obtained directly under a light bulb, it may look a bit darker with the LED bulbs.
In order to solve this problem, FIG. 4 of PTL 1 discloses an LED bulb with an expanded illumination angle, in which a substrate is formed in a cylindrical shape, LEDs are mounted on both a side surface (surface parallel to the axis of the LED bulb) of the cylinder and an upper surface (surface perpendicular to the axis of the LED bulb) of the cylinder, and the light emitted from the LEDs is diffused by a fluorescent material layer formed in the inner surface of a translucent cover and is emitted to the outside of the LED bulb.